1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a “laminated media” magnetic recording disk with improved intrinsic media signal-to-noise ratio (S0NR).
2. Description of the Related Art
In magnetic recording disk drives, where the magnetic recording media on the disks is a granular metal alloy, such as a CoPt alloy, the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the readback signal peaks. High media noise leads to high bit error rates. Thus to obtain higher areal densities in magnetic recording disk drives, it is necessary to decrease the intrinsic media noise, i.e., increase the signal-to-noise ratio (S0NR), of the recording media.
The media S0NR is to first order proportional to 20log (N1/2), where N is the number of magnetic grains per unit area in the media and S0NR is expressed in units of dB. Accordingly, increases in S0NR can be accomplished by increasing N. However, N is limited by the individual grain area (A) required to maintain the thermal stability of the recorded magnetization. This limitation arises because the energy term protecting against thermal degradation is KUV, where KU is the anisotropy and V is the volume of an individual magnetic grain. KUV must be kept greater than a certain value to assure thermal stability of the recorded magnetizations. Increasing N by merely reducing the grain area A will reduce V since V=At, where t is the grain height (i.e., the thickness of the magnetic recording layer), and this will reduce KUV, leading to thermal instability. One approach to prevent this problem is to proportionally increase the anisotropy KU as V is decreased. However, this approach is limited by the available magnetic write field produced by the recording head. The magnetic field necessary to write the media (i.e., change the recorded magnetizations) is represented by the short-time or intrinsic coercivity H0 of the media, which is proportional to KU/M, where M is the grain magnetization or magnetic moment. Therefore, increasing KU will increase H0 and may prevent the media from being able to be written by a conventional recording head. Thus, to ensure reliable operation of a magnetic recording disk drive, the media must have sufficiently high S0NR, sufficiently low H0 to be writable, and sufficiently high KUV to be thermally stable.
Improved media S0NR can be achieved with “laminated” media. In laminated media, the single magnetic layer is replaced with a laminate of two or more separate magnetic layers that are spaced apart and magnetically decoupled by nonmagnetic spacer layers. This discovery was made by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706–2709, and patented in U.S. Pat. No. 5,051,288. Published patent application U.S. 2002/0098390 describes a laminated media of two or more magnetic layers wherein the lower magnetic layer is an antiferromagnetically-coupled (AFC) layer. Laminated media increases S0NR because N is increased, e.g., essentially doubled when two magnetic layers are used or tripled when three magnetic layers are used. In laminated media the same magnetic alloy composition that was used in the single magnetic layer is used in all magnetic layers, so that it is not necessary to use a higher KU magnetic alloy material. Thus KU remains the same as for the single-layer media. If each magnetic layer in the laminate is also the same thickness as the single magnetic layer, then the grain volume V remains the same because the grains in the two magnetic layers are magnetically decoupled by the nonmagnetic spacer layer. Thus S0NR is increased without a reduction in KUV so that thermal stability is not decreased.
However, the laminated media approach to increasing media S0NR requires substantially thicker media, e.g., a doubling of the total magnetic layer thickness if two magnetic layers are used. An increase in the total thickness causes a different problem, namely difficulty in writing. This is because the write field from the recording head decreases with distance from the write head and thus the strength of the write field is less at the bottom magnetic layer than at the top magnetic layer. If H0 of the bottom magnetic layer in the laminated media is greater than the write field, the magnetization of the bottom magnetic layer cannot be switched and thus data cannot written to the laminated media. Thus it has not been possible to fabricate useful laminated media with more than two magnetic layers.
What is needed is laminated media with more than two magnetic layers and with good magnetic recording properties.